Properties of Aromatic Compounds
Aromatic compounds, originally named because of their fragrant properties, are unsaturated hydrocarbon ring structures that exhibit special properties due to their aromaticity, including an unusual stability. They are often represented as resonance structures containing single and double bonds. However, the bonding is stronger than expected for a conjugated structure, and it is more accurately depicted as delocalized electron density shared between all the atoms in the ring, as shown for benzene in Figure 1.
Aromatic compounds are cyclic structures in which each ring atom is a participant in a pi bond, resulting in delocalized pi electron density on both sides of the ring. Because of this connected network of pi bonds, the rings are planar unlike cycloalkanes.
Aromatic compounds are generally nonpolar and immiscible with water. As they are often unreactive, they are useful as solvents for other nonpolar compounds. Due to their high ratio of carbon to hydrogen, aromatic compounds are characterized by a sooty yellow flame.
The double bonds in aromatic compounds are less likely to participate in addition reactions than those found in typical alkenes. Because of pi electrons, aromatic compounds can act as Lewis bases, and they often participate in electrophilic substitution reactions. Aromatic compounds can also react in nucleophilic aromatic substitution reactions.
Aromatic compounds are produced from a variety of sources including petroleum and coal tar. Poly-aromatic hydrocarbons are components of atmospheric pollution and are known carcinogens. Aromatic compounds are also of interest because of their role in the origins of life as precursors to nucleotides and amino acids.